1. Field of the Invention
The invention relates to radiological image intensifier tubes, and more particularly to means for improving the image resolution of these tubes.
2. Description of the Prior Art
An image intensifier tube is a vacuum tube comprising an input screen, located at the front of the tube, an electronic optical system and a screen for the observation of the visible image located at the rear of the tube, on the same side as an output window of this tube.
In X-ray or radiological image intensifier tubes, the input screen furthermore has a scintillator screen which converts the incident X photons into visible photons.
FIG. 1 gives a schematic view of a radiological type of image intensifier tube such as this.
The radiological image intensifier tube 1 comprises a glass envelope 2, one end of which, at the front of the tube, comprises an input screen 3 exposed X photon radiation.
The second end of the envelope forming the rear of the tube is closed by an output window 4 transparent to light.
The X-rays are converted into light rays by a scintillator screen 5. The light rays excite a photocathode 6 which produces electrons in response.
The electrons produced by the photocathode 5 are accelerated towards the output window 4 by means of different electrodes 7, and by an anode 8 positioned along a longitudinal axis 13 of the tube and forming the electronic optical system.
The output window 4 is formed by a transparent glass part which, in the example shown, bears a cathodoluminescent screen or output screen 10, made of luminophores for example.
The impact of the electrons on the cathodoluminescent screen or output screen 10 makes it possible to reconstitute an image (amplified in luminance) which was initially formed on the surface of the photocathode 6.
The image displayed by the output screen 10 is visible through the glass part which constitutes the output window 4. Generally, optical sensor devices (not shown) are positioned outside the tube in the vicinity of the output window 4 to pick up this image through the window 4 and enable it to be observed.
In the most recent observations, the input screen 9 comprises an aluminium substrate covered by the scintillator 5, itself covered by an electrically conductive and transparent layer 11, made of indium oxide for example. The photocathode is deposited on this transparent layer 11.
The X-rays strike the input screen on the aluminium substrate side. They go through this substrate and then reach the material constituting the scintillator.
The light photons produced by the scintillator are emitted in about every direction. However, to increase the resolution of the tube, the scintillator material chosen is generally a substance such as caesium iodide (CsI) which has the property of growing in the form of crystals perpendicular to the surface on which they are deposited. The needle crystals thus deposited tend to guide the light perpendicularly to the surface, which favors high image resolution.
The French patent application No. 88.09938 dated Jul. 22, 1988 describes the way to improve this resolution by reducing the mean cross-section of the needle crystals of the scintillator, through the surface condition of the layer on which the scintillator is made to grow.
The quality of the image resolution may also be lowered because light photons generated in the scintillator start off again towards the side on which the X-rays arrive. These photons strike the aluminium substrate with an incidence that is random. They are reflected by the aluminium substrate frontwards, hence towards the photocathode, but the path of these photons is such that the result is a loss of resolution: for a same X-photon incidence, it is possible to arrive at a situation where electrons are created at points in the photocathode that are different from those required.
FIG. 2 gives a view, in greater detail, of the input screen 9 and illustrates this loss of resolution by showing, side by side, the different paths followed by two light photons PL1, PL2 arising out of the impact of an X photon on the scintillator 5, resulting in the formation of electrons at different points of the photocathode. The input window 3, through which the X-rays arrive, constitutes the aluminium substrate bearing the cesium iodide scintillator 5, the crystals 5a of which are perpendicular to the surface and tend to channel the light photons. The transparent conductive sub-layer referenced 11 is positioned between the scintillator 5 and the photocathode 6.
In the example shown in FIG. 2, the light photon PL2 is emitted backwards, i.e. towards the substrate 3, with an incidence such that it is reflected by the substrate towards the photocathode 6, the path that it takes in the scintillator 5 being a needle crystal different from the one in which it has been generated: this fact illustrates the loss of resolution.